The flue gases of a conventional power station typically can contain from about 3% (by volume) to about 15% carbon dioxide (CO2) along with other components that can be captured prior to being vented to the atmosphere. An efficient method of capturing CO2 from flue gases so as to produce a stream of CO2 that can be readily transported to a safe storage site or to a further application such as an enhanced oil recovery operation would be useful. CO2 has been captured from gas streams by four main technologies. Reactive absorption is used to selectively capture the CO2 into a liquid absorbent such as an amine based process. Membranes, such as semipermeable plastics or ceramic membranes, are also used to separate CO2 from the gas streams. Adsorption is another method for separating CO2 from a gas stream. Adsorption involves the disposition of CO2 on the surface of specially designed, high surface area solid particles. The CO2 can then be separated using an adsorption-desorption cycle. Finally, a low temperature/high pressure process can be used to separate CO2 from a gas stream, where the separation is achieved by condensing the CO2 from the flue gas stream.
Separation technologies used with these process include, for example, distillation columns, stripping columns, electrostatic precipitators, and dryers. These units are generally large in size, have long residence times, and require high energy consumption. In addition, these devices are relatively inefficient in separating gaseous mixtures. For example, in such systems, the separation of CO2 from a gas stream may be performed using a series of stripping columns to remove the CO2 followed by the distillation of the solvent to recover the CO2. The equipment involved usually requires a large footprint due to the numerous pieces of process equipment needed for such a separation scheme. Such a process may also suffer from high energy consumption requirements and solvent loss during operation.
Current flue gas CO2 separation methods focus on capturing CO2 by scrubbing the flue gas with an amine solution. This technology is used commercially for small scale flue gases, but its application decreases the total efficiency of the power plant by a significant amount. In addition, amine scrubbing still requires that the flue gas stream be cleaned of impurities (e.g., sulfur, nitrogen oxides, and particulates) in order to minimize contamination of the solvent. For example, distillation columns, stripping columns, electrostatic precipitators, and dryers may be required in addition to the amine unit.
A traditional amine plant involves contacting a gas stream comprising a target component (e.g., CO2) with a reactive absorbent in a stripping column. The gas removed from the stripping column is clean gas with the majority of the target component removed. The reactive absorbent is traditionally an amine that forms a complex with a target component such as carbon dioxide. The target component enriched complex then passes to a regenerator tower, which may be a stripping column or distillation tower, where the enriched complex is heated to release the target component. Additional equipment required to operate the amine unit typically includes flash tanks, pumps, reboilers, condensers, and heat exchangers. When the gas stream contains a large concentration of a target component, the energy required to remove the target component may exceed the useful chemical energy of the stream. This limitation sets an upper concentration level of the target component at which the process can be economically run. This process also suffers from a high energy consumption, solvent loss, and a large footprint, making the process impracticable for uses requiring a small footprint.
Filtration and membrane separation can also be used to remove components of a power plant flue gas. These processes involve the selective diffusion of one gas through a membrane to effect a separation. The component that has diffused through the membrane is usually at a significantly reduced pressure relative to the non-diffused gas and may lose up to two thirds of the initial pressure during the diffusion process. Thus, filters and membrane separations require a high energy consumption due to the compression required for the feed mixture and re-compression for the gas diffusing through the membrane. In addition, membrane life cycles can vary due to plugging and breakdown of the membrane, requiring additional downtime for replacement and repair.
Alternative separation processes such as cyclones can be used to separate gases. Cyclones utilize centrifugal force to separate gaseous components from gas-liquid fluid flows by way of turbulent vortex flow. Vortices are created in a fluid flow so that heavier particles and/or liquid droplets move radially outward in the vortex, thus becoming separated from gaseous components. Considerable external energy must be added to cyclones to achieve effective separation.
Centrifuges and cyclones both use centrifugal force to achieve separation. Centrifugal separators can achieve separation of immiscible or insoluble components from a fluid medium; however, centrifugal separators require mechanical acceleration of up to 20,000 G. The mechanical parts and energy needed to achieve these velocities make centrifugal separators costly to operate to effectively remove components from a fluid.
U.S. Pat. No. 6,524,368 (Betting et al.) refers to a supersonic separator for inducing condensation of one or more components followed by separation. Betting is directed to the separation of an incompressible fluid, such as water, from a compressible fluid. In this process, a gas stream is provided to the inlet. The gas converges through a throat and expands into a channel, increasing the velocity of the gas stream to supersonic velocities. The expansion of the flow in the supersonic region results in incompressible fluid droplets which are separated from the compressible gas. The system involves a significant pressure drop between the inlet and outlet streams, and a shock wave occurs downstream after the separation, which may require specialized equipment to control.
In a thesis by van Wissen (R. J. E. van Wissen, Centrifugal Separation for Cleaning Well Gas Streams: from Concept to Prototype (2006)), gas centrifugation is described for separating two compressible fluids in the absence of an incompressible fluid. The separation is carried out using a rotating cylinder to create a plurality of compressible streams based on the difference in the molecular weight of the gaseous components. As noted in the thesis, the potential to separate compressible components such as carbon dioxide from light hydrocarbons is limited by the differences in molecular weights between the components. As such, centrifuges cannot provide a highly efficient separation when the component molecular weights are close to one another. Such a design also suffers from an extremely low separation throughput rate that would require millions of centrifuges to handle the output of a large gas source.
A turboexpander is an apparatus which reduces the pressure of a feed gas stream. In so doing, useful work may be extracted during the pressure reduction along with a corresponding temperature drop in the stream. Furthermore, an effluent stream may also be produced from the turboexpander. This effluent may be passed through a separator or distillation column to separate the effluent into a heavy liquid stream. Turboexpanders utilize rotating equipment, which is relatively expensive. Such equipment requires a high degree of maintenance and, because of the moving parts, has a higher incidence of mechanical breakdown. In addition, turboexpanders are poorly suited for certain applications, such as for feed gas streams with entrained liquids or liquids that form upon a pressure or temperature drop. In this instance, the liquid droplets can damage the rotating blades due to the high impact forces between the droplets and the moving blades.
What is needed is a separation apparatus and method that provides high separation efficiency of compressible components while avoiding or reducing pressure drop, and the need to supply large amounts of external energy.